1. Field of the Invention
The present invention is directed to photobioreactors and their use in the production of biomass, and more particularly to photobioreactors with rotating and/or oscillating lighting and mixing systems that provide delivery of light to all areas of the photobioreactors.
2. Description of the Related Art
Algae have been cultivated artificially for such diverse purposes as the production of food for animals and humans, the treatment of sewage and waste waters, and the accumulation of radioactive wastes. More recently, algal cultures have been used for the production of enzymes having industrial and research applications and for producing oils and other materials having value as fuels such as biodiesel fuel or ethanol. However, production of biodiesel and ethanol fuels can be expensive and inefficient processes. It would be desirable to increase the efficiency of production of these materials in order to meet rising world demand.
Photobioreactors are known in the art, and are generally used for the production of biomass. Photobioreactors generally consist of a vessel containing a liquid medium that is exposed to a light source. However, the configuration of the photobioreactor often prevents the light from penetrating more than a few centimeters from the surface of the liquid. This problem reduces the efficiency of the photobioreactor, and was recognized in “Solar Lightning for Growth of Algae in a Photobioreactor” published by the Oak Ridge National Lab and Ohio University:                Light delivery and distribution is the principle obstacle to using commercial-scale photobioreactors for algae production. In horizontal cultivator systems, light penetrates the suspension only to 5 cm leaving most of the algae in darkness. The top layer of algae requires only about 1/10th the intensity of full sunlight to maximize growth, so the remaining sunlight is wasted.        
According to Mario R. Tredici: “Outdoors, under full sunlight, the photosynthetic efficiency drops to one tenth-one fifth of the values observed at low irradiances. The major causes for this inefficiency are the light saturation effect (LSE) and photoinhibition, phenomena that strongly limit the grows of microalgae in out door culture, although these because of the high cell density, are light-limited. The main problem is that photosynthetic apparatus of phototrophs saturates at low irradiances (typically from 1/20 to 1/10 of full sunlight) and that, at irradiances above saturation, the absorbed photons are used inefficiently and may cause cell injury. Several strategies to overcome the LSE and photoinhibition have been proposed, based on engineering (light dilution, ultra high cell density culture, high turbulence), physiologic (photoacclimation, nutrient deprivation) or genetic . . . .” (Tredici M. R. (2004) Mass production of microalgae: photobioreactors. In Richmond A (ed.), Handbook of Microalgae Culture. Blackwell Publishing, Oxford (UK), pp 178-214.
As described in Healthy Algae, Fraunhofer Magazine, January 2002, “Algae are a very undemanding life form—they only need water, CO2, nutrients and sunlight. However, providing sufficient sunlight can be a problem in large scale facilities. “As the algae at the surface absorb the light, it does not penetrate to a depth of more than a few millimeters. The organism inside the unit gets no light and cannot grow,” explains Walter Troesch, who has been cultivating algae for years. “This is the reason why there are only a few Algae production units dotted around the world. One of the problems with growing algae in any kind of pond is that only in the top ¼” or so of the pond receives sufficient solar radiation for the algae to grow. In effect, this means that the ability of a pond to grow algae is limited by its surface area, not by its volume.”
Algae are a useful organism. Algae contain fat, carbohydrates, and protein. Some of the micro-algae contain up to 60% fat. Once the fat is ‘harvested’—some 70% can be harvested by pressing, and what remains becomes a good animal feed or can be processed to produce ethanol, according to “Cultivating Algae for Liquid Fuel Production” by Thomas F. Riesing, Ph.D.
Closed photobioreactors are known for use in the production of biomass. See, for example, U.S. Pat. No. 5,151,347 to Delente et al. One type of photobioreactor is a tubular type, such as the industrial tubular photobioreactor that was established near Wolfsburg, Germany in 2000. This particular tubular photobioreactor has a total tube length of approximately 500,000 meters, and a total volume of approximately 700 cubic meters. Total annual production of biomass from this photobioreactor is estimated to be approximately 130-150 tons of dry biomass in an area of approximately 10,000 square meters (O. Pulz, IGV Institute for Cereal Processing, Arthur-Scheunert-Allee 40/41, 14558 Bernholz-Rehbrucke, Germany). Another type of closed photobioreactor is a tank with immersed light-emitting tubes. These types of photobioreactors were considered in patents bellow.
Approaches have been described to increase the efficiency of the photobioreactors by using artificial light sources. For example, U.S. Pat. No. 6,602,703 discloses a photobioreactor having light-emitting tubes mounted within the container. U.S. Pat. No. 5,104,803 discloses a photobioreactor for the cultivation of photosynthetic microorganisms having at least one light bank substantially totally immersible in the liquid microbial culture. U.S. Pat. No. 5,614,378 discloses a photobioreactor system that includes a fiber optic based optical transmission system that illuminates the reactor internally and includes a light source which is external to the reactor.
In addition, one problem with available photobioreactors is that any liquid microbial culture will receive light only near the top of the reactor, or near the lighting system. As the culture becomes more dense, light has difficulty penetrating this dense microbial culture. The result is that only the lighted space is effective for growing the microbial culture, and this leads to inefficiencies in the production system.
Although many photobioreactors have been proposed in the prior art, there is still a need for an improved photobioreactor using artificial light so that the efficiency of these devices may be increased. The present invention is believed to be an answer to that need.